Feasibility of 2 °C as a post-2020 warming threshold via input-constrained optimal control

Governments around the world have committed to urgent reductions in anthropogenic emissions of greenhouse gases (GHGs) in order to limit the increase in global average temperature to less than 2 °C relative to pre-industrial levels, a warming level recognized as a key threshold for dangerous climate change. In the lead-up to a key United Nations Framework Convention on Climate Change (UNFCCC) meeting in Paris in late 2015, countries have begun to announce GHG emission reduction pledges for the period beyond 2020. In this paper, we investigate feasibility of 2 °C as a post-2020 warming threshold via the solution of an input-constrained optimal control problem in which the control inputs to be optimized are the emissions reduction rate, and the fraction of net economic output invested in capital. Using the DICE (Dynamic Integrated model of Climate and the Economy) integrated assessment model (IAM), we show that rapid emissions reductions are necessary to reach global net zero emissions by mid-century and so restrict warming to 2 °C. Attempts to limit warming to 2 °C by constraining either the absolute rate, or rate of growth, in emissions reductions effort in a global post-2020 mitigation scheme are achievable only with appreciable loss of economic output, with total emissions falling sharply as a consequence. The results of this paper underscore the importance of recognizing the interdependence of geophysical and economic dynamics in attempting to stay below the 2 °C warming threshold.